Revealing CO-Preferential Encapsulation in the Mixed CO-N2 Clathrate Hydrate

In addition to their various applications in the environment, energy and technology fields, gas clathrate hydrates, called gas hydrates (inclusion compounds composed of a H-bonded water network forming cages, inside of which gaseous guest molecules are trapped), are hypothetically involved in the formation of planetesimals, comets, and other planets (e.g., the Saturn's moon or the Jupiter-family comets). In these issues, CO and N-2 molecular species often occur. In this work, we have elucidated the molecular selectivity and the guest partitioning of the CO and N-2 molecules within mixed CO-N-2 gas hydrate (the two guest species are encapsulated within the cages) as a function of the gas mixture composition at 270 K and 200 bar. By means of micro-Raman spectroscopy, it is shown that CO molecules are preferentially encapsulated within the hydrate cages, with a better selective efficiency for gas mixture below 35% CO in N-2 gas. In addition, the present Raman investigation gives clear indications about the formation of both SI and SII clathrate structures, depending on the gas mixture composition: below 35% CO, the mixed hydrate is mainly of type SII, whereas above this limit, the formed mixed hydrate is mainly of type SI. Moreover, no preferential CO/N-2 fractioning in the various types of hydrate cages is observed. In addition to the interest for gas separation technology (e.g., flue gas), such experimental results corroborate, for the first time, theoretical and modeling prediction, showing that the formation of clathrate hydrate could explain the potential N-2 depletion observed in Jupiter-family comets (e.g., 67 P/Churyumov-Gerasimenko).